With the growth of highly sophisticated information society, development of a microwave band is promoted and demand for highly sophisticated microwave components is increasing. As one of the above, there is a broadband variable attenuator which has a broad band in a high-frequency range and of which an attenuation amount is adjustable.
For example, as a broadband variable attenuator used in a microwave band, there are known a T-variable attenuator constituted by connecting field effect transistors (FETs) in T-shape and a π-variable attenuator constituted by connecting field effect transistors (FETs) connected in π-shape. Further, a variable attenuator is suggested in which switching between T-shape and π-shape is possible by controlling a gate voltage of the FET and so forth (for example, see Japanese Patent Application Laid-open No. Hei 6-112767).
For the broadband variable attenuator, a good input/output characteristic and a large attenuation amount are required. However, in a conventional broadband variable attenuator, it is quite difficult to simultaneously obtain two characteristics of the good input/output characteristic and the large attenuation amount.
FIG. 10 is a diagram showing a circuitry of a conventional variable attenuator. A variable attenuator 100 includes transmission lines 3a, 3b, 3c, and 3d connected in series between an input terminal 1 and an output terminal 2. The transmission lines 3a to 3d are transmission lines whose line lengths are quarter wavelength (λ/4).
Also, the variable attenuator 100 includes FETs 4a, 4b, and 4c functioning as variable resistance elements and adjusting an impedance (alternating-current resistance) in the variable attenuator 100, that is, an attenuation amount by the variable attenuator 100. The FETs 4a to 4c are provided in a manner to correspond to respective interconnection points (between 3a-3b, between 3b-3c, and between 3c-3d) of the transmission lines.
Drains of the FETs 4a, 4c are connected to the interconnection points between the transmission lines 3a-3b and 3c-3d via resistance elements 101, 102. A drain of the FET 4b is connected to the interconnection point between the transmission lines 3b-3c. Sources of the FETs 4a to 4c are connected to the ground (are earthed). Gates of the FETs 4a to 4c are connected to a control terminal 6 via resistance elements 5a to 5c respectively.
The resistance elements 101, 102 are interposed in order that an input/output reflection characteristic is improved to obtain a good input/output characteristic in the variable attenuator 100. Resistance values (impedances) thereof are Z0 (for example, about 50 ohm, respectively).
FIG. 11 is a diagram showing an equivalent circuit at a time of maximum attenuation of the conventional variable attenuator 100 shown in FIG. 10. At the time of maximum attenuation, the FETs 4a to 4c are turned on by control voltage supplied via the control terminal 6 (assume that on-resistance values are RON).
On this occasion, as shown in FIG. 11, in addition to the on-resistance values RON of the FETs, the resistance values Z0 of the resistance elements 101, 102 are applied to between a signal line constituted with the transmission lines 3a to 3d and the ground. Accordingly, in the signal line, the impedance from a viewpoint of a node N11 becomes large enough, but the impedance from a viewpoint of a node N12 does not become large because of an influence of the resistance element 102, and so the attenuation amount cannot be made sufficiently large.
In other words, as shown in FIG. 10, when the variable attenuator is constituted in a manner that the resistance element is interposed in series between the signal line and the ground for the sake of acquisition of the good input/output characteristic, the interposed resistance element suppresses increase of the impedance in the signal line. As a result, the attenuation amount (attenuation capability) in the variable attenuator is deteriorated and a large attenuation amount cannot be obtained.